Method of and apparatus for noise measurement or indication in an electric circuit



Oct. 21, 1958 J. R. ALTIERI 2,357,531

METHOD OF AND APPARATUS FOR NOISE MEASUREMENT R INDICATION IN ANELECTRIC CIRCUIT Original Filed May 14, 1953 Sheets-Sheet l CONSTANT l fURRENT POTENTIOMETER VOLTAGE SOURCE UNDER TEST AMPLIFLIER l l 12 j 13NEON ONE-SHOT GATE INDICATOR MULTIVIBRATOR CIRCUIT 1 15 1 g-1.

' MK POWER 18p AMPLIFIER N0-e0 REGION |000\ U (I) 900 74 2 e00 LIJ g 700v s00 REGION y Q 500 S 400 5; g; 300 m E 200 o I IllHll lllllll Illlllll I lllHll 2 small 2 345e7' h 2 a4ss1|| 2 a45s7| 9 9 9 9 I0 I I00I000 |o000 100000 Y INCIUDED RESISTANCE(OHMS) IN VEN TOR.

AM 12.4%,, 0 l l 1 THRESHOLD RESTSTANGE (onms) y /fozwgg AMBIGUITY(OHMS) Oct. 21, 1958 J. R. ALTIERI 2,857,531

METHOD OF AND APPARATUS FOR NOISE MEASUREMENT OR INDICATION IN ANELECTRIC CIRCUIT Original Filed May 14, 1953 5 Sheets-Sheet 2 INVENTOR.

Oct. 21, 1958 J. R. ALTIERI v2,857,531

METHOD OF AND APPARATUS FOR NOISE MEASUREMENT OR INDICATION IN ANELECTRIC CIRCUIT Original Filed May 14, 19 53 3 Sheets-Sheet 3 l a? J'fINVENTOR.

United States I Patent METHOD OF AND APPARATUS FOR NOISE MEASUREMENT ORINDICATION IN AN ELECTRIC CHKCUIT Joseph R. Altieri, Beverly Hills,Califi, assignor, by mesne assignments, to Acton Laboratories, Inc.,Acton, Mass.

Original application May 14, 1953, Serial No. 355,104, now Patent No.2,778,994, dated January 22, 1957. Divided and this application October8, 1956, Serial No. 614,486

3 Claims. (Cl. 307-149) The present invention relates to a method of andapparatus for determining the equivalent noise resistance in a network,and more particularly to noise resistance of a movable contact of apotentiometer.

This is a division of my copending patent application, Serial No.355,104, filed May 14, 1943, now Patent No. 2,778,994, for a Method ofand Apparatus for Noise Measurement or Indication in an ElectricCircuit.

A precision potentiometer comprises a resistor having at least twoterminals, one at each end. Some potentiometers have intermediate tapsbut for the present purposes they need not be considered. A movablecontact engages intermediate points along the resistor. Noise appearsbetween the moving contact and the resistor which has a resistancecharacteristic, although transient in character. Noise therefore appearselectrically as the third resistance element of a Y network. This hasbeen defined as the equivalent noise resistance. It is the purpose ofthe present invention to measure or indicate the transient peak value ofthe equivalent noise re sistance as the third leg of a Y network whereinthe resistor of a potentiometer comprises two legs of the Y network. I

Noise is of fundamental importance in the physical and engineeringsciences in considering communication and the utilizing of intelligence.Noise places a limit on the useful operating or dynamic range ofelectronic and electromechanical mechanisms. In general internal noiseis that portion of the output of any system, not originally present inthe input signal, and not directly attributable to specificallyprescribed operations on the input by the system.

In the manufacture of precision potentiometers the noise developed as adirect result of the actuation of the potentiometer shaft carrying themovable contact is of great importance. The importance of noisedeveloped by precision potentiometers has heretofore been the subject ofvarious investigations. In one investigation an attempt was made todiscover a wire and contact material combination which would reduce thenoise output as well as enhance the longevity of a potentiometer inmotor driven applications. This investigation used as a criterion forthe noise characteristic of a potentiometer, the widening of aresolution pattern of a cathode ray oscillograph for observing a motordriven potentiometer as compared to the voltage output of a standardmaster precision potentiometer. the potentiometer under investigationand the master potentiometer was supplied to a very high impedanceamplifier for operation of an oscillograph. This observation of thewidening of the resolution pattern was a satisfactory indication ofpotentiometer noise only when it was integrated into a system havinginput characteristics similar to the amplfier used in the test. The useof amplifiers of his type, however, was not feasible for productiontesting. i

Another system for attempting toisolate noise generation in a precisionpotentiometer was predicted on the The difference output between ICCassumption that for all practical purposes the contact arm on thewinding was either prefect or was essentially open circuited as a resultof foreign material lodged on the wiper track of the resistance element.For this purpose an oscillograph was employed and reliance was placed onthe widening of the time trace of the oscillograph base. In a variationof this system the potentiometer was excited from a voltage source and aresolution pattern was observed which depended upon the sweep frequencyof the oscilloscope and the speed of rotation of the potentiometer.Neither of these variations gave any quantitative indication of thenature of the noise characteristic of the potentiometer underinvestigation. 4

An attempt to deal quantitatively with the noise characteristic was madeby exciting the potentiometer with a specified voltage through aspecified series resistance. The output of the potentiometer was appliedto an oscilloscope on which less than the specified number of voltageswere taken as a criterion to determine satisfactory and unsatisfactorypotentiometers. Such an arrangement, however, has the disadvantage inthat the sensitivity of the test varies with the position of the contactarm on the winding, with the total resistance of the potentiometer, withthe series resistance and with the magnitude of the voltage source. Ithas the further disadvantage common'to all oscilloscope tests forpotentiometer noise, in that it requires the human operator to discernalmost small instantaneous flashes in the oscilloscope pattern which arenot periodic, and which are of relatively small luminous intensity. Ifthe sensitivity of the system is increased, this difiiculty becomesgreater because the largest part of the indication of a noise pulse mayfall beyond the oscilloscope screen which may or may not be observed bythe operator.

Noise in potentiometers is perhaps primarly of two types, active andpassive. Active noise may be said to consist of voltages appearing atthe output terminals of a potentiometer even though the potentiometer isnot excited when actuated. Such noise may be due to thermal electriceffects, heating of dissimilar metals as when precious metal contactsare supplied over a resistance wire upon operation of the rotor, changesin the relative work function as the sliding contact shifts from onetype of resistance wire to another or from the terminating travelportion to the resistance winding, and due to chemico-electric effectsresulting from moisture contact and residual, chemical or soft materialson the winding element.

Passive noise in a potentiometer includes contact resistance variationsresulting from variations in contact pressures, and discontinuities incontacting surfaces when the sliding contact is moved from one part ofthe winding to another part. It also may be due to a lack of homogeneousspecific resistance at the surface or skin of the wire of the resistanceelement as a result of local crystallization or oxidation, or it may bedue to embedded foreign material in the winding.

Hence it is apparent that noise in a potentiometer may result from amultiplicity of causes. In commercial production of potentiometers itwould appear that passive noise might be more susceptible of control inthe manufacturing process. It further becomes evident that noise in aprecision potentiometer has for some time eluded quantitatively definesnoise in terms of an equivalent noise resistance. While such definitionmay not be subject to profound mathematical analysis such as thatemployed in determining the equivalent noise resistance relative otatomic motion, the equivalent is of convenience to bring a multiplicityof noise causes down to a common level so that they may be subject toquantitative evaluation.

Therefore, noise may be defined quantitatively in terms of an equivalentparasitic, transient, contact resistance in ohms appearing between thewiper contact arm and the resistance winding of a precisionpotentiometer when the wiper arm is actuated. The equivalent noiseresistance is defined independent of the total resistance, theresolution, the physical characteristic, the total travel, and the speedof operation of the wiper arm of the potentiometer. The magnitude of theequivalent noise resistance is taken as the peak value obtained. Theslider or contact arm of the potentiometer is arbitrarily required to beexcited by a current of 1.0 milli-ampere or the data corrected to referto this standard condition.

In the above definition, one milli-ampere exciting current is specifiedto provide a common basis for standard measurements. This figure isquite arbitrary, and should not be considered to becompletelyrestrictive. It is evi dent that it may prove desirable tomake measurements at some other value of current. There are many reasonsfor this; only'three of which will-be outlined below. Additional reasonswill come to mind to individuals skilled in this art.

There is evidence to indicate that the value of the equivalent noiseresistance may depend upon the magnitude of the exciting current. Thisappears particularly true in regard to the passive component of theequivalent noise resistance. It has been discovered that in some casesthis passive component possesses a current sensitivity as a non-linearcharacteristic, and for certain applications, it may be desirable toascertain the nature of this non-linear characteristic by makingequivalent noise resistance measurements, e. g., from 1 micro-ampere tomilliamperes, and plotting the equivalent noise resistance values soobtained.

An additional reason for desiring to make the equivalent noiseresistance determination at some value other than 1 milli-ampere may beevident to those. skilled in the art of computer designand potentiometerspecification standardization when consideringtherelation of theinfluence of the active and passive components of the equivalent noiseresistance of the'potentiometer on the system under consideration.equivalent noise resistance usuallyhas its peakmagnitude (threshold ofacceptable performance) determined by the amount of gain presentfollowing the potentiometer element and by the relationof the designedoutput voltage of the potentiometer tothe magnitude of the activeportion of the equivalent noise resistance present. This ratio is a typeof signal-to-noise ratio.

The passive portion of the equivalent noise resistance usually has itslargest usable peak magnitude established in terms of the amountof ohmicresistance present in the circuit which is excited by the potentiometer.In some cases, in addition to this,'the threshold of acceptance isestablished by the relation between the amount of spurious A. C. pickupin the circuit excited by the potentiometer at high impedance levels andby the recovery time characteristic ofthe succeeding amplifiers whensaturated by the aforementioned stray pickup.

In the light of these considerations,-it is evident that the fullestflexibility of'the equivalent noise resistance definition may beobtained and-a tremendous practical advantage simultaneously accruedinregard to'simplifying Production and Engineering Testing ofpotentiometer assemblies by assigning arbitrarily a value of excitingcurrentin conjunction with the equivalentnoise resistance The'activeportion of the L 4 determination, which will cause the samemagnitude of disturbing effect upon the system by the active componentsand the passive components of the equivalent noise resistance, actingalone.

Thirdly it is evident that by assigning a very large value of excitingcurrent during the equivalent noise resistance determination, thepassive components of noise may be determined in peak magnitudeessentially independently of the active noise present. Similarly, byarbitrarily requiring the exciting current to be zero, the activecomponents of the potentiometer noise may be ascertained essentiallyindependent of the passive noise present.

Having defined the equivalent noise resistance, it now would bedesirable to provide an apparatus particularly suited for determiningthe equivalent noise resistance in ohms independent of the totalresistance of the potentiometer or circuit, the physical characteristic,the total electrical angle, the resolution, and the speed of operation.While the speed of operation may affect the equivalent noise resistancemeasured in a potentiometer, it can, however, become invariant byexcluding resolution and the speed of operation.

In accordance with the present invention there has been provided asystem for determining the equivalent noise resistance ofpotentiometers, of Y-connected networks and contacts. To eliminatesubjectiveness of the test where the human operator may or may notdiscern small or excessively large extremely transient noise variations,a visible and an audible signal of predetermined magnitude are providedupon the occurrence of a parasitic or transient variation in the contactresistance, contact voltage, or a combination thereof, in excess of apredetermined magnitude.

It, therefore, is an object of the 'present'invention to provide asystem of measurement of the equivalent parasitic transient contactresistance between a contact arm or slider and the resistor element of apotentiometer.

A still further object of the invention is to provide a system fordetermining this equivalent noise resistance in an electric circuit.

A further object of the invention is to "provide asystem for themeasurement of equivalentnoise resistance in potentiometers and variableresistors.

A still further object of the invention is toprovide a system for themeasurement of equivalent noiseresistance between two electric contacts.

A further object of the invention is to provide .a go, no-go tester forchecking potentiometers and variable resistors with respect to apredetermined noise level.

Still another object of the invention is to provide a system for themeasurement of the magnitude of the equivalent noise resistance in anelectric network or in a potentiometer.

Still another object of the invention is to provide a potentiometernoise tester which will give uniform indications, visual and oral,independent of the transient characteristic of the noise resistance.

A further object of the invention is to provide a peak reading, voltagesensitive circuit for testingor determining the electric variations inelectric contacts.

Still another object of the invention is to provide a peak'reading,voltage sensitive circuit for testing potentiometers for equivalentnoise resistance.

A further object of the invention is to provide a method for testingpotentiometers and eliminating subjective characteristics in testing.

Still another object of the invention is to provide a method formeasuring contact resistance of a slider or contact arm of apotentiometer independently of itsposition on its resistance element,or'the resistance value of the resistance element.

A further object of the invention is to provide an improved method formeasuring the contact resistance-of a slider or contact arm of apotentiometerirrespective of its position on the resistance element, andirrespective of the linearity, non-linearity or total resistance valueof the resistance element.

A further object of the invention is to provide a method of positiveindication of definite duration independently of the transient nature ofnetwork noise whenever a predetermined peak value is exceeded.

A still further object of the invention is to provide a method of andapparatus for measuring the resistance of one leg of a Y-connectednetwork irrespective of the resistance values of the other two legs ofthe network.

Other and further objects of the invention subsequently will becomeapparent by reference to the following description taken in conjunctionwith the accompanying drawings wherein:

Figure 1 is a block diagram of the system of the present invention fortesting Y-connected networks and potentiometers;

Figures 2a and 2b are detailed circuit diagrams of the systemillustrated in Figure 1;

Figure 3 is a circuit diagram of a standard equivalent noise resistancesynthesizer circuit;

Figure 4 is a graph showing a typical sensitivity characteristic of thecircuit of Figure 1; and

Figure 5 is a graph showing the typical accuracy and precision resultsto be obtained by the use of the method and system herein described.

Referring to Figure 1 of the drawings it will be seen that the blockdiagram shows that the system contemplated in according with the presentinvention employs a constant current source 11 which is connected to apotentiometer 12 which is under test. ,The potentiometer 12 is alsoconnected to a voltage amplifier 13 having its output connected tocontrol a gate circuit 14. The gate circuit in turn controls a one shotmulti-vibrator 15 which controls or actuates a neon indicator 16 thusproviding a visual indication upon the occurrence of a certain conditionor whenever a predetermined condition has been exceeded. The one shotmulti-vibrator 15 is also connected to control a tone oscillator 17which is coupled through a power amplifier 18 to a loud speaker 19. Theoperation of the one shot multi-vibrator therefore produces a visualindication by control of the oscillator 17 which is amplified by thepower amplifier 18 and reproduced as a tone by the speaker 19. The oneshot multi-vibrator provides an indication of known duration independentof the transient characteristic of the equivalent noise resistance. Thegate circuit 14 establishes the go, no-go characteristic set for theparticular potentiometer under test and also the width of the zone ofambiguity.

The block diagram of Figure 1 is embodied commercially in a singleinstrument having three input terminals which are connected to the endterminals of a potentiometer resistor and to the movable contactterminal. The shaft of the potentiometer then is rotated. If the movablecontact or wiper at any instant makes imperfect contact, the operatorwill be notified by a visual indication and an audible indication. Theseindications are of a predetermined duration and do not depend upon theduration or magnitude, beyond the set threshold, of the transientimperfect contact known as noise. Any potentiometer tested, whichresults in such indication, falls into the no-go classification.

Before explaining in further detail the operation of the presentinvention it is believed that reference may better be had to Figure 2 sothat those skilled in the art may appreciate the circuit details of thesystem set forth in block diagram in Figure l. The constant currentsource 11 employs a pentode vacuum tube 21 having a fixed cathoderesistor 22 and an adjustable cathode resistor 23 so that the value ofthe constant current to be supplied may be adjusted. The screen grid ofthe vacuum tube 21 is connected to a circuit which includes a resistor24 having one end connected to ground, a neon lamp 25 shunted by aresistor 26 and a series resistor 27 connected to a conductor 30extending to a resistor which is con nected to a terminal 29 in a socket31. The neon bulbeludes, in the maximum case, the total resistance ofthe potentiometer 12 under test and the contact resistance in the slidercircuit. The neon lamp 25 therefore pro-- vides positive indication whenthe contact of the potentiometer under test is off the active portion ofthe potentiometer element. Furthermore, the capacitor 32 provides for acontrolled amount of integration so that this indicator circuit is notactuated by exceedingly large noise impulses of short duration. Asuitable by-pass capacitor 32 is connected between ground and the screengrid. The control grid of the vacuum tube 21 is connected through aresistor 33 to a portion of the voltage supply system for the apparatus.The anode of the vacuum tube 21 is connected through a shieldedconductor 34 to the contact 35 of a socket 36. The contact 37 of thesocket 36 is connected through a shielded conductor 38 to one terminalof the resistor 27 forming a part of the circuit to which the screengrid is connected. Another contact 39 of the socket 36 is connectedthrough a shielded conductor 41 to a coupling capacitor 42 which isconnected to the grid of a triode 43 which forms a part of the amplifier13.

The constant current supplied by the source 11 which includes the vacuumtube 21 is connected to the potentiometer 12 under test by means of aplug 36a having a contact 35a which engages the female contact 35. Thecontact 35a is connected to one terminal of the resistance element ofthe potentiometer 12. The contact 37a is connected to the movablecontact or wiper arm of the potentiometer 12. The remaining contact 3911is connected to the other terminal of the resistance element of thepotentiometer 12.

The grid circuit of the vacuum tube 43 of the amplifier 13 is providedwith a gridto ground resistor 44. The cathode is provided with a cathodeto ground resistor 45. The anode of the vacuum tube 43 is provided withan anode resistor 46 having an adjustable contact 47. One terminal ofthe resistor 46 is connected to the common juncture between theresistors 27 and 28, and this common juncture is also connected to acapacitor 48 having its other terminal connected to ground.

The amplifier 13 also includes a second triode portion 49 having itsgrid connected through a coupling capacitor 51 to the adjustable contact47 of the resistor 46. In the go, no-go instrument contemplated by thepresent invention the adjustable contact 47 is set by a screw driveradjustment within the cabinet of the apparatus. If it were desired tomeasure the particular value of contact resistance, the contact 47 couldbe connected to a suitable indicating dial thus to provide an actualindication of the resistance value. The grid of the triode portion 49 ofthe amplifier 13 is provided with a grid to ground resistor 52 and acathode is provided with a grid to ground resistor 53 which is by-passedby a capacitor 54. The anode of the triode 49 is provided with an anodecoupling capacitor 56 to the grid of the triode 57 forming a portion ofthe gate circuit 14. The cathode of the triode portion 57 is connecteddirectly to ground. The anode of the triode portion 57 is connected tothe grid of a second triode portion 58 and to an anode resistor 59. Agrid resistor 61 is connected to one terminal of the anode resistor 59and which extends to a contact 63 in the socket 31.

The socket 31 has a contact 64 connected by conductor 60 to anindicating lamp 66. The socket 31 also has a contact which is grounded.The contact 64a is connected to a contact 67 arranged to be engaged bythe switch blade 68 of a foot switch 69. The contact arm or switch blade68 is connected to the contact 65a which is grounded at 65 and to oneend of a resistor 71. The other end of the resistor 71 is connected to acontact 72 which is manually engaged by a switch blade or arm 73 whichis connected to the contact 29a. When the foot switch 69 is actuated,the two switch blades 68 and 73 are moved downwardly so that the switchblade 73 engages a contact 74 which is connected to contact 63a thuscompleting a circuit between the conductor 62 and one terminal of theresistor 28. The closing of the switch 68 completes a circuit betweenthe grounded contact 65 and the contact 64 which is connected to theindicating lamp 66 which becomes illuminated to indicate that the footswitch has placed the apparatus in operation. The lamp 66 has oneterminal connected to one end of a transformer secondary winding 75having its other end connected to ground. The transformer winding 75also supplies power to an electric circuit including an indicating lamp76 and a series resistor 77. The lamp 76, therefore, is illuminatedwhenever the transformer of which the secondary winding 76 is a part hasbeen energized. The foot switch 69, therefore, merely completes acircuit so that current can be applied to the potentiometer under test,and during the times that the switch 69 is not being operated thepotentiometer may be handled without fear of any shock or injury to theoperator.

The cathode of the triode portion 53 of the gate circuit 14 is connectedto a conductor 78 which is connected to the high voltage sourceemploying two voltage regulator tubes 79 and 81. The anode of the triodeportion 58 is coupled through a capacitor 82 to the triode portion 83 ofthe multi-vibrator 15. The anode of the triode portion 58 is providedwith an anode resistor 84 having one end connected to the commonjuncture between a resistor 85 having one end connected to ground and aresistor 86 having one end connected to the cond uctor 62 which isby-passed to ground by a capacitor 87. The conductor 62 is connectedthrough a resistor 88 to a conductor 89 which leads to the anoderesistor 55 of the preceding amplifier triode 49 of the amplifier 13.The resistor 85 is provided with a by-pass capacitor 91.

The grid of the triode portion 83 of the multi-vibrator 15 is connectedto a coupling capacitor 92 to the anode of a triode portion 93 of themulti-vibrator 15. The grid of the triode portion 83 is connectedthrough a resistor 94 to the conductor 89. The anodes of the triodeportions 83 and 93 are connected through anode resistors 95 and 96respectively to the conductor 89. The cathodes of the triode portions 83and 93 are connected to a cathode to ground resistor 97. The grid of thetriode portion 93 of the multi-vibrator 15 is connected to a movablecontact 98 of a resistor 99 which is connected by the series resistors181 andliiZ. Thus the circuit including the resistor 99, resistors 101and 102 extends between the grounded conductor and conductor 89. Theanode of the triode portion 83 of the multi-vibrator .15 is connected tothe grid of a triode portion 103 of the neon indicator circuit 16 whichhas its anode connected directly to the conductor 89. The cathode of thetriode portion 103 is connected to a circuit which includes a seriesresistor 104 connected to a parallel circuit including a resistor 105parallel to a circuit comprising a resistor 106 in series with a neonlamp 107 which in turn is connected to the grounded conductor. The neonlamp 107 becomes illuminated whenever the triode portion 103 responds tothe one shot multi-vibrator 15. The anode of the triode portion 103 isconnected to a bypass capacitor 108 which in turn is connected toground. The cathode of the triode portion 103 is connected through aresistor 109 to the anode of another triode 111 which is a portion ofthe oscillator circuit 17. The

cathode of the triode portion 111 is connected to ground through aresistor 112 which is by-passed by a capacitor 113. i The grid of thetriode 111 is connected to one terminal of a resistor 114 which isconnected to a resistor 115 having its terminal connected to a capacitor116 which is connected to the anode of the triode 111. The commonjuncture between resistors 114 and 115 is connected through a capacitor117 to ground. The juncture between the resistors 115 and 116 isconnected to one terminal of a capacitor 118 which is connected toanother capacitor 119 having its terminal connected to the grid of thetriode 111. The common juncture between the capacitors 118 and 119 isconnected through a resistor 121 to ground. The common juncture betweenthe capacitor 116, the resistor 115 and the capacitor 118 is connectedto one terminal of a resistor 122 having its other terminal connected toground and being provided with an adjustable contact 123 which isconnected to the electrode of a pentode 124. The screen grid of thepentode 124 is connected through a resistor 125 to the conductor 89. Thescreen grid is also connected directly to a conductor 12-0 which isconnected to the high voltage end of the voltage regulator tube of thepower supply. The cathode of the pentode 124 is connected through abiasing resistor 126 to ground. The anode of the pentode 124 isconnected through the primary winding 127 of a transformer 128 to theconductor 125. The secondary winding 129 of the transformer 128 isconnected to the voice coil 131 of the loud speaker 19.

The apparatus thus far described is arranged to be energized from asuitable source of alternating current 132 connected through a switch133 to a transformer 134 having parallel connected primary windings 135and 136. A secondary winding 137 supplies filament current to arectifier 138 which has its anodes connected to the other terminals of acenter tapped winding 139 forming the remaining secondary of thetransformer 134. The cathode of the rectifier 138 is connected to oneside of a filter circuit which includes a resistor 141, a choke coil 142and a resistor 143. Filter capacitors 144 and 145 interconnect oppositeterminals of the choke coil 143 with a conductor 146 which is connectedto the mid tap of the transformer secondary Winding 139. The two voltageregulator tubes 79 and 81 are connected between the one terminal ofresistor 143 and the conductor 146. In parallel with the voltageregulator tubes is a resistor circuit including a resistor 147 and aresistor 148. The resistor 148 has an adjustable contact 149 which isconnected by a capacitor 151 to the conductor 146 which is grounded. Theadjustable contact 149 is connected to one terminal of the secondarywinding 137 and to a conductor 152 forming a part of a filter circuit.Connected across the winding 137 is a resistor 153 having an adjustablecontact 154 connected through a unilateral conductor device or rectifier155 to a filter circuit which includes a resistor 156 connected to aconductor 157 which in turn connects to one terminal of the gridresistor 33 of the triode 31 of the current source 11. The commonjuncture between the resistor 156 and the rectifier 155 is connected toa capacitor 158 which in turn is connected to the conductor 152. Theother terminal of the resistor 156 is connected to a capacitor 159 whichin turn is connected to the conductor 152. A resistor 161 is connectedbetween the conductor 152 and one terminal of the resistor 156. Theconductor 157 is connected to one terminal of a capacitor 162 which inturn is connected to ground.

The adjustments provided by the contacts 154 and 149 serve to establishthe adjustment of the current source stabilizing circuit, comprisingresistors 153, 147, 148, 161, 156, and 33; conductor 157, non-linearimpedance 155 and capacitors 151, 158, 159 and 162, with transformerwinding 137. This circuit makes possible very accurate steady statecompensation for the quiescent operating conditions of the pentode 21 soas to cancel out shifts in operating characteristics resulting fromvariations in heater voltage under conditions of varying power linevoltage; thereby enabling the maintenance of constant current throughthe contactor 35 and the potentiometer under test to a very precisedegree.

The apparatus comprising the system set forth in the circuit diagram ofFigure 2 is primarily intended to be used for production testing ofpotentiometers. An apparatus of this type may be .employedadvantageously for an engineering analysis of the performance ofpotentiometers specifically, and electric contacts in general,considerations of design and development as well as for engineeringevaluation of potentiometer performance in a system in terms of itsnoise characteristics. Several variations and elaborations of the systemset forth in the design of Figure 2 have already been investigated andinclude such variations as additional amplification to allow operationat lower equivalent noise resistance levels, calibrated attenuation andamplification to facilitate convenient adjustment of the go, no-gothreshold, elaboration of the current source to permit operation at awide variety of currents, automatic drive mechanisms to control theoperation of the potentiometer actuator, and elaboration of the circuitinvolving jack 36 to simultaneously enhance shielding for low leveloperation and minimization of stray capacity. Heretofore it has beencommon to define an unsuitable potentiometer by the expression that ithas an open. In the past to some engineers the term open would mean acompletely open circuit or an essentially infinite resistance betweenthe potentiometer winding and the adjustable contact or slider. Suchconcept is satisfactory where the precision potentiometer is applied ina circuit where the slider or contact arm is connected to the gridcircuit of a cathode follower or other high impedance device. It is nowevident that this concept is merely a degenerate form of the verygeneral concept of equivalent noise resistance propounded by thisinventor for the case where the equivalent noise resistance is very,very large (equals infinity or essentially so). This limited degeneratecase, however, has proved itself unsatisfactory in those potentiometerapplications where the slider or contact arm is loaded or connected toanother potentiometer as in the case of analogue computer multiplyingcircuits, or by a fixed resistor when the shaft of the precisionpotentiometer is being rotated. An open for the purposes of the presentinvention is defined as a malfunctioning characteristic of a precisionpotentiometer involving a parasitic transient resistance between thewiper arm and the actual point of contact on the resistance elementwinding when the wiper arm is being actuated. To be strictly correct,however, the use of the term open should be restricted to refer toequivalent noise resistances of exceedingly large magnitude as mentionedabove. Determination of such malfunction should be entirely independentof the potentiometer characteristic, the speed of the test, and theoperator. For certain purposes it is desirable to limit the equivalenttransient resistance to a particular ohmic value. In order to adjust thecircuit shown in Figure l to a particular standard it is convenient toemploy a standard open or standard equivalent noise resistancesynthesizer shown in circuit diagram in Figure 3. It will be noted thata male plug similar to 36: shown in the circuit diagram of Figure 2 maybe employed to connect the standard open to the circuit of Figure 2. Apotentiometer having a resistance element 171 may be connected to thecontacts 35a, and 39a. When this potentiometer is employed, the positionof its slider synthesizes the slider position of an actual potentiometerunder test by inserting into the current source leg, or the measurementcircuit leg, a value of series resistance to be synthesized. Theoperation of the circuit of Figure 2 is adequately trustworthy so thatthe resistance element 171 is not required when the galibration of thecircuit of Figure 2 is the only purpose '10 for which the synthesizer isemployed. In such cases the contacts 35a and 39a may be joined by aconductor and joined to the wiper mm 172, in the diagram of theequivalent noise resistance synthesizer of Figure 3. In many otherapplications, the resistance element 171 may be important; one suchapplication will be described later. The adjustable contact or wiper arm172 is connected through two circuits, one of which includes anadjustable resistor 173 to the contact 37a. The other circuit betweenthe contact 37a and the contact arm 172 includes a pair of electricswitches arranged in parallel. These switches have conductive segments174 and 175 provided with a gap of any convenient length. Two contactarms 176 and 177 are connected to a common actuating shaft and soarranged that these contact arms may be displaced relative to each otherin a range from .05 to 2.0". It will be noted that the two contact arms177 and 176 serve to short circuit the adjustable resistor 173 for apredetermined portion of the total actuator position which may beadjusted by changing the relative position of conductive segments 174and 175. The adjustment of the resistor 173 which conveniently may be acalibrated rheostat or a decade box, determines the value of theresistance which may appear between the contact 37a and the wiper arm172 of the standard potentiometer. The decade box may be set within arange from ten ohms to approximately five thousand ohms. By connectingthe standard equivalent noise resistance synthesizer shown in Figure 2,the adjustable contact 47 on the resistor 46 of the amplifier 13 may beadjusted so that no signal is produced when the wiper arms 176 and 177are caused to rotate through the gap of the conductive segments 174 and175 and the adjustable resistor 173 is set to a value slightly less thanthe desired threshold value. The operation of this circuit may beobserved from Figure 2. On the other hand, when the actuator of contacts176 and 177 is rotated through the aforementioned gap, a signal shouldbe produced for adjustments of the resistor 173 equal to or greater thanthe desired threshold value for the circuit of Figure 2. The speed ofoperation of the switch arms 176 and 177 would correspond to the speedof rotation of a potentiometer under test. The angular duration of theopen circuit condition of these switches corresponds to the angularduration of a simulated equivalent noise resistance condition. Hence itis possible to adjust the circuit of Figure 2 to determine the range ofambiguity. When the resistor 173 is adjusted to a value greater thanthat desired for the go, no-go condition there should be no ambiguity inthe operation of the circuit of Figure 2 since it should consistentlyindicate the presence of an open circuit. When the resistor 173 is setsomewhat less than the critical value, the circuit of Figure 2 shouldnever give any indication irrespective of the rotation of the switcharms 176 and 177. In the particular embodiment contemplated the zone ofambiguity is found 1" (1.5% of the adjusted value of the resistor 173two ohms).

This relation with respect to ambiguity is illustrated graphically inFigure 4 wherein ambiguity expressed in (1- ohms) was plotted acrossthreshold resistance (ohms). It is believed that those skilled in theart will require no further explanation of the meaning of this graphicalrepresentation.

The utility of the equivalent noise resistance synthesizer of Figure 3as an instrument to facilitate the adjustment of the go, no-go thresholdof the circuit of Figure 2 as well as to make possible a determinationof the Zone of ambiguity has been discussed. An interesting applicationfor the apparatus of Figure 3 for eX- perimentally determining the peakmagnitude of the passive portion of the equivalent noise resistance, ofany potentiometer, that will produce a predetermined amountof'disturbing effect on a system under investigation will now beoutlined.

The resistance element 171 of Figure 3 is connected into the system asthe potentiometer under investigation, and contactor 37a is connected tothe system as the slider. The actuator of contacts 176 and 177 isoperated at the same speed as the actuator for contact 172. Thecalibrated rheostat or decade box 173 is adjusted to various values ofequivalent noise resistance to be simulated and the system responseevaluated by an engineer, technician, or other person skilled in theart. When a predetermined amount of disturbing effect on the systemobtains the threshold magnitude (level of acceptability) of theequivalent noise resistance appearing on the calibration of rheostat 173is noted.

It is clearly evident that disturbing effect of the active portion ofthe equivalent noise resistance or any combination of active and passiveportions may similarly be synthesized.

Still another way of expressing the operation of the circuit of Figure 2and the zone of ambiguity is shown by the graph of Figure 5 wherein thethreshold resistance has been shown in ohms as against the includedresistance in ohms. Thus for a particular setting of the circuit ofFigure 2 all resistance values below the zone of ambiguity A wouldappear to pass inspection, and all resistance values which exceed thezone of ambiguity A would be greater than that which has been consideredto be permissible, and hence such devices would be rejected. Wheneverthe circuit of Figure 2 detects a condition exceeding the zone ofambiguity A of Figure 5, both audible and visual signals will occur. Inthe zone of ambiguity the circuit of Figure 2 may or may not responddependent upon various factors, but the circuit is sufficiently accuratethat it is positive that there will be no response when the effectiveresistance value is anything below the lower edge of the zone ofambiguity A.

The accuracy and reliability of the indication provided by the circuitof Figure 2 is not appreciably affected by the speed of rotation of thepotentiometer slider or contact arm for very low speeds up to about 60R. P. M. It furthermore is insensitive to the total resistance of thepotentiometer provided it is less than 100,000 ohms, and the linearityor non-linearity characteristic of the potentiometer, and the totalelectrical angle of rotation or resolution. Perhaps a fullerappreciation of the nature of the apparatus provided by the circuit maybe had by further consideration of its mode of operation. Thepotentiometer 12 under test is excited by current applied between thecontact 35a and the conductor 37a of the plug 36a. Thus constant currentof a known magnitude flows in the potentiometer winding independent ofits total resistance, function, linearity or nonlinearity, or theinstantaneous position of the slider or contact arm. The other end ofthe potentiometer winding and the slider or contact arm is locked in bya peak indicating voltmeter to provide the oral and visual indicationwhen the peak voltage appearing between contact 39a and 37a of the plug36a exceeds a predetermined threshold magnitude. The peak indicatingvoltmeter has a characteristic so that its indication is essentiallyindependent of the widths of the voltage pulses supplied to it, and thusit provides a determination of the continuity characteristic of thepotentiometer under test independent of the angular space occupied bythe zone of discontinuity. The amplifier 13 which is connected to thisportion of the potentiometer under test has an essentially infiniteinput impedance which does not influence the measurement, and hence onlythe total transient voltage developed between the actual point ofcontact of the slider of the potentiometer on the winding and the slideris observed. It has previously been indicated that the circuit has acharacteristic that the zone of ambiguity about its threshold adjustmentis a minimum. The circuit is further designed to re-set itself aftereach indication within approximately 0.5 second or less recovery time.The indication provided upon 12 the occurrence of any transient voltagein excess of a predetermined amount in the potentiometer portion whichis connected to the amplifier 13 produces an indication forapproximately three seconds. This indication is provided by the neonlamp 107 and the audible signal from the loud speaker 19.

Where 360 mechanical rotation potentiometers are to be tested which arecompletely encased so that the internal construction and operationcannot be observed during the test, the neon indicator lamp 25 would beextinguished when the intended electrical continuity of the winding isexceeded. The threshold resistance value of the circuit may be pre-setto any desired value between 10 ohms and 5,000 ohms by adjustment of thecontact 47 on the variable resistor 46 of the amplifier 13.

Vhile for the purpose of simplicity in describing the present inventionreference has been had to the use of the circuit and apparatus for thepurpose of testing otentiometers, it, of course, will be appreciatedthat the apparatus is capable of other uses. It will readily beappreciated that the potentiometer 12 under test shown in Figure 2actually constitutes a Y-connected network. Hence any other Y-connectednetwork could be substituted for the potentiometer 12 under test. Suchan arrangement would therefore make it possible to test contacts such asthose used in relays or mechanically operated switches. Thus it ispossible to determine the noise introduction of contacts in anyelectrical circuit arrangements where the contacts are eitherelectrically or mechanically actuated as a part of the operationsequence.

It further will be appreciated that while the foregoing description hasbeen directed to a go, no-go type of operation which defines thegreatest utility in production testing operations, that the circuit andapparatus is not limtied to such use. It previously has been indicatedthat an absolute measurement of the slider or contact resistance couldbe obtained by the use of a suitable calibration dial connected to theadjustable contact 47 of the resistor 46 of the amplifier 13. From thisit will be appreciated that further refinements or modifications mightbe made whereby a suitable attenuator or phase reversal mechanism mightbe introduced for balancing out the active noise components in aY-connected network. It further will be appreciated that while in theparticular embodiment referred to it has been stated that thesensitivity could be set within a range from 10 ohms to 5,000 ohms thatsuitable amplification might be provided to extend the range as low as0.01 ohm. The system, therefore, is believed to be useful in locatingthe erratic operation in servo-mechanisms and analogue computers as wellas other circuit devices wherein the reliability of contact might be thefactor introducing the erratic operation or be the factor in reducingthe over-all sensitivity of the system.

While for the purpose of illustrating and describing the presentinvention certain specific components have been referred to,particularly in connection with the description of Figure 2, it is to beunderstood that the invention is not to be limited thereby since suchother components and such variations in the circuit arrangements arecontemplated as may be commensurate with the spirit and scope of theinvention set forth in the accompanying claims.

I claim as my invention:

1. An equivalent noise resistance synthesizer for standardizingapparatus for testing a potentiometer to determine the magnitude of theequivalent noise resistance thereof, said synthesizer comprising a threeterminal network having the resistance element of a potentiometerbetween two of said terminals, and a circuit connected between thecontact arm of said potentiometer and the remaining terminal of saidnetwork comprising a variable resistor, and a pair of rotary switchesconnected in 13 parallel to said variable resistor and adjustablerelative to each other to determine the angular effectiveness of saidvariable resistor.

2. An equivalent noise resistance synthesizer for standardizingapparatus for testing a potentiometer to determine the magnitude of theequivalent noise resistance thereof, said synthesizer comprising a threeterminal network having the resistance element of a potentiometerbetween two of said terminals, and a circuit connected between thecontact arm of said potentiometer and the remaining terminal of saidnetwork comprising a variable resistor, and a pair of rotary switches inparallel thereto having contacts arranged adjustable relative to eachother to determine the angular eifectiveness of said variable resistorduring rotation of said switch contacts.

3. For use with apparatus employed to measure the equivalent noiseresistance of a potentiometer and comprising a constant current sourceand a peak indicating voltmeter, an equivalent noise resistancesynthesizer by means of which said apparatus may be standardized toprovide an indication only when the equivalent noise resistance of apotentiometer under test exceeds a particular ohmic value, said noiseresistance synthesizer comprising a network having a first terminalwhereby said network may be connected to one side of said constantcurrent source, a second terminal whereby said network may be connectedto one side of said voltmeter, and a third terminal whereby said networkmay be connected to the other sides of said constant current source andsaid peak indicating voltmeter, a potentiometer comprising a resistanceelement and a movable contact arm, said resistance element beingconnected between said first and second terminals, a circuit connectedbetween said contact arm and said third terminal, said circuitcomprising a variable resistor connected in parallel with a pair ofrotary switches also connected in parallel to each other, said rotaryswitches having switch arms adjustable relative to each other, saidswitches also having conductive segments each provided with a gapwhereby as said switch arms are rotated said switches may be opencircuited relative to said variable resistor.

No references cited.

